In general, metal containers are broadly divided into two-piece cans and three-piece cans. The two-piece can refers to a metal container consisting of two parts of a can body integral with a can bottom and a lid body. The three-piece can refers to a metal container consisting of three parts of a can barrel, an upper lid, and a bottom lid. Although the can body of the two-piece can has a fine appearance because it has no seamed part (welded part), it in general requires a high degree of working. In contrast, although the can barrel of the three-piece can is inferior in appearance as compared to the can body of the two-piece can because it has seamed parts, it does not in general require a high degree of working. Given these circumstances, there is a tendency that the two-piece cans are used for expensive small-volume metal containers and the three-piece cans are used for inexpensive large-volume metal containers.
Among the two-piece cans, as a metal material for a can body of a two-piece can that has a high degree of working in drawing and has a high degree of stretching in a can height direction, that is, a two-piece can having a high degree of working, a soft metal material such as aluminum, which is expensive and has a large sheet thickness, is used, and a steel sheet such as tinplate or tin-free steel, which is inexpensive and has a small sheet thickness, is little used. The reason is that while forming methods having a high degree of working such as the drawing method and the draw and ironing (DI) method are hard to be adopted to the steel sheet, the impact forming method having a high degree of working can be applied to the soft metal material. Examples of the two-piece can having a high degree of working include an aerosol can.
For a two-piece can having a low degree of working, technologies have been developed for manufacturing can bodies by the drawing method and the DI method using as a material a resin coated metal sheet in which a metal sheet has resin coating layers on both sides thereof (see Japanese Examined Patent Application Publication No. 7-106394, Japanese Patent No. 2526725 and Japanese Patent Application Laid-open No. 2004-148324). To allow treatment to improve the designability of a can body such as printing treatment, technologies have also been developed to add a white pigment to a resin coating layer positioned outside a metal container after forming (Japanese Patent Application Laid-open Nos. 8-169098 and 2004-130536).
We investigated whether a can body of a two-piece can having a high degree of working can be manufactured using a steel sheet that is inexpensive and has high strength despite of its small sheet thickness, a two-piece can having a high degree of working can be provided at a lower price. We then manufactured a can body of a two-piece can having a high degree of working using a resin coated metal sheet and performed heat treatment at a temperature close to the melting point of the resin coating layer to increase adhesion between the resin coating layer and a metal sheet after forming, the laminatability of the back side resin coating layer positioned inside a metal container after forming, and the designability of the front side resin coating layer positioned outside the metal container after forming. As a result, we found that a black-spot shaped pattern was formed on the front side resin coating layer after heat treatment and that appearance defects were caused by the heat treatment. Given this situation, to manufacture a can body of a two-piece can having a high degree of working using a resin coated metal sheet, it is required that no appearance defect is caused by heat treatment.